The invention relates to sound generators typically used in computing devices. In particular, the invention relates to a compact sound generator circuit for a personal digital assistant. Yet further still, the invention relates to a circuit configured to improve the sound quality of a simple sound generator for a handheld computer.
Handheld computing devices, xe2x80x9cpalmtopsxe2x80x9d, xe2x80x9cpalmheldsxe2x80x9d, personal digital assistants (PDAs), or handheld computers typically weigh less than a pound and fit in a pocket. These handhelds generally provide some combination of personal information management, database functions, word processing, and spreadsheets. Because of the small size and portability of handhelds, strict adherence to hardware constraints, such as sound generation hardware, must be maintained. It is conventional to use a sound generator in a handheld device which is configured to operate ideally at a particular single frequency, rather than across a broad audio frequency range. When the sound generator is used across the audio frequency range, it provides xe2x80x9cpoor sound qualityxe2x80x9d with a widely varying sound pressure level (SPL) over the audio frequency range for the same user setting.
Other conventional implementations of sound generation circuits include a dynamic speaker that is designed to operate across an audio frequency range having a substantially flat frequency response across the range. Such dynamic speakers are physically larger and cost many times more than sound generators. Further, the dynamic speaker drive circuit is also more complicated and expensive to implement than a simple sound generator.
Further, as depicted in FIG. 4 an audio system 5 of the prior art includes a resistor and capacitor (RC) circuit 10 and is used with a pulse width modulated (PWM) signal source 20, a fourth order low pass filter 30, and an audio amplifier 40 to provide audio signals to a speaker 50. Low pass filter 30 typically causes substantial attenuation of the filtered PWM signal from source 20. Accordingly, audio amplifier 40 is required to provide suitable amplification for the audio signal. Thus, because of the complexity of a fourth-order low pass filter, and the requirement for an amplifier, a substantial expense is associated with the circuit components of system 5 and substantial space requirements are necessary for housing the circuit components of system 5 within a handheld computer. Also, the amplifier requires additional power which may not be available in a handheld device. Further, system 5 requires utilizing a reconstruction rate of the PWM output of 32 kHz. Such rapid sampling, along with associated table look-ups and calculations, requires additional processing power and speed which may be unavailable on a handheld device.
Accordingly, there is a need for a compact sound generator circuit that utilizes simple circuitry to improve sound quality over an audible frequency range. Further, there is a need for a sound generator circuit that is used to provide improved sound quality using a compact sound generator and utilizing a PWM signal.
The teachings herein below extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the above mentioned needs.
An exemplary embodiment relates to a sound generator circuit configured for a handheld computer. The sound generator circuit includes a low pass filter having an input and an output. The input is coupled to a pulse width modulated (PWM) signal line. The sound generator circuit also includes a switching circuit coupled to the output of the low pass filter and being controlled by the output of the low pass filter. Further, the sound generator circuit includes a sound generator having a first and a second terminal, the first terminal being coupled to a voltage source and the second terminal being coupled to a switching circuit. Furtherstill, the sound generator circuit includes a high pass filter coupled across the first and second terminals.
Another exemplary embodiment relates to a handheld computer. The handheld computer includes a processor, a memory coupled to the processor, and a sound generator circuit configured to receive a pulse width modulated (PWM) signal from the processor. The sound generator circuit includes a low pass filter having an input and an output. The input is configured to receive the PWM signal. The sound generator circuit also includes a switching circuit coupled to the output of the low pass filter and being controlled by the output of the low pass filter. The sound generator circuit further includes a sound generator having a high pass filter coupled to and in parallel therewith.
Further, an exemplary embodiment relates to a method of producing a tone with a sound generator in a handheld computer. The method includes filtering a pulse width modulated (PWM) signal with a low pass filter to provide a slope to the edges of the PWM signal. The method also includes causing a switching circuit to open and close according to the filtered signal. Further, the method includes filtering the signal using a capacitor disposed across the terminals of a sound generator.